Method of producing chemical conversion coatings on aluminum surfaces



METHOD .OF. PRODUCING CHEMICAL CONVER- SION COATINGS N ALUMINUM SURFACESNelson J. Newhard, .Jr.,- Oreland, David Y. Dollman,

1 Lansdale Mounted Route, and Lester Steinbrecher, Philadelphia, Pa.,assignors to Amchem Products, Inc., Ambler, Pa., a corporation ofDelaware :F No Drawing. Filed Nov; 19, 1959, Ser. 'No. 854,005

5 Claims. (Cl. 148-62) 1 This invention relates to the art of producingchemical conversion. coatings on the surface of aluminum or alloys.thereof in which aluminum is the principal or predominant ingredientand isprimarily concerned with aluminum coating. processes inwhich the,surface of. the metal is treated with acidic solutions containingfluorine bearcompounds and hexavalent chromium. The invention isparticularly useful. in. thetreatment of large areas of aluminum surfacein relatively short periods of time and particularly under conditions ofheavy'bath loading, i.e.,

Where large surface areas are treated per unit volume of bath per unitof time.

In situations of this kind it is most important that the bathor-solutionwhich is used in the process be kept in proper operating condition andthe presentinvention has for its primary object the provision of certainimprovements in the replenishing technique whereby the bathmaybeHmaint-ained in suitable condition for the *rapid production of usefuland uniform coatings both as to color andcorrosion resistance even insituations where the bath is being subjected to heavy loading.

The nature and advantages of the invention will be better understood ifcertain prior art practices are'kept. in

mind. Forinstance, it is wellknown that highly. corrosion-resistant as-well as decorative coatings can be .produced on. aluminum surfaces bytreating them with acidic solutions containingfluorine bearing compounds:and hexavalent chromium as their principal and essential ingredients.(It should be noted at this point that in the present disclosureWherever the term fluorine bearing compound appears it is used to meanany compound containing an atom of fluorine in its makeup whether or notthe fluorine present is in the form of a simple or complex fluoride.) Itis' alsowell known that coatings pro ice inc, bearing compounds andhexavalent chromium. Industrially, bathsofthetype described have becomeextremelyimportant'and there are a number of materials commerciallyavailable by means of which the solutions. may be initially prepared andsubsequently replenished. Examples of these which are well known to ,thetrade are sold under the trademarks Alodine, I1idite and Bonderite. TheAlodine solutions for this purpose are known as Alordine 600 and Alodiue1200, the Iridite isv known as Iridite 14, and Iridite 14-2 and theBonderite is known as Bonderite 721, and all of these consist primarilyand essentially of hexavalent chromium, a fluorine bearing compoundcontaining either a simple or a complex salt, and a cyanide selectedfrom .the class consisting of ferro and .ferricyanides, togetherwithmineral acids. as may be necessary to produce the desiredrange of pHin the coating bath.

The practices of the art towhich reference has been made have quiteoften resulted in the production of coatings which are non-uniform inboth appearance and corrosion resistance, especially where the coatingsolutions are employed in the treatment of a long succession of aluminumsurfaces in relatively short periods of time.

p The present invention overcomes the previous difliculties and is basedupon the discoverythat, if coating solutions of the type described arereplenished from time to time by reston'ng'the hexavalent' chromiumcontent to the desired level and, further, that if the fluoride and thefern'cyanide are-added to the bath in a definite relaing such solutions,it has been customary, if necessary, to

add from time to time suflicient mineral acid such as nitric acid tomaintain the coating solution at the desired degree of acidity. 7 I

Unfortunately, the foregoing practices of the prior art have not alwaysbeen successful in maintaining the ability of the coating bath toproduce the desired degree of uniformity in coating, especially inprocesseswhich involve the treatment of large areas of aluminum inrelatively vshort periods of time. This problem has been experienced toa marked degree in strip line production where the coating baths aresubjected to unusually heavy loading.

The. compositions of coating solutions of the character described arewellknown in the art, and by way of -.example reference may be had toUnited States Patents 2,276,353; 2,507,956; 2,851,385; 2,796,370 and2,796,- 371. It will be. noted from the last two patentsdisted that thepresenceof ferro or. ferricyanide ions-:inthe .bath significantlyimproves the coating characteristics of a; bath which consistsprincipally and .essentiallyjfof fluorship to the amount of hexavalentchromium that is added, it is possible to maintain the bath in suchcondition that it will produce uniformly. colored and corrosionresistant -coatings even in situations where a long succession ofalurminum surfaces are treated over relatively-.short.periods of time.Furthermore, in replenishing the fluoride-conl tent, we have found thatit is necessary-to employ .a simi ple fluoride regardless of whether-.or. not .the. bath was originally made up or formulatedwithcomplex or.simple fluorides. Specifically, in the present invention, the =-fluoridewhich is added to the bath must be addedin the form of hydrofluoric acidor an alkali or-ammoniumsalt I of such acid and theaddition must be madein a quantity from 0.4 to 2 parts of the fluoride in the form justmentioned for each part of chromate vadded (calculated -as CI'Og) Whenthesource of fluoride is hydrofluoricacid, best results are secured byadding betweenv 0.5 and 0.75 part of. fluoride. for .eachpartofchromate. In general, if hydrofluoric acid isthessource of thefluorideynot more than" .8.-part F shouldabe added for each partofhexavalent" chromium. Furthermore, where the fluorideiis .introduced byadditions of alkali or ammonium salts, bestresults are secured by adding0.75 part to parts of salt-for each partof. chromate added. If lesszthan0.5 'part fluoride is added :for each part of hexavalent l chromium(calculated as CrO it will benoted that' the uniformity of thecoatings'produced by thebathrv'vill be rapidly impaired or no visiblecoatings whatsoever will be produced. When the amount offluoride,added-t'is more than 1 to 2 parts for every part of hexavalentichromiumadded, thecolor of the coating gradually-becomes lighter and lighteruntil no visible coating is produced.

In-situa-tions where rather brilliant colors are desir'ed in the coatingthebath shouldalso contain ferricyanide and in replenishing thisingredient there should be added t least 005 part offerricyanide andpreferably .from 0.2 to 1 part of ferricyanide for each part'of chromateV which is added to, the bath. .When less than 0.05 part; offerricyanide. is added; the color-imparted to 'the coating :will, overvaygiven-period of time, become lighter and lightersand theucoatingweight ;;will graduallydiminish.

Although we have just indicated a maximum of 1 part as a potential upperlimit on the quantity of fern'cyanide added we wish to say that suchlimit is dictated by reasons of economy rather than by necessity becausethe use of more than 1 part does not seem to harm the process or theresults attained in any way.

Obviously, it is essential that the acidity of the invention becontrolled. This may be done in whole or in part by replenishment of thehexavalent chromium content as chromic acid and of the fluorine contentas hydrofluoric acid or a simple salt thereof. As a general practice thepreferred method of controlling the bath pH is through the use ofchromic acid (CrO and hydrofluoric acid, but in situations where this isnot effective an amount of mineral acid, preferably nitric acid, may beadded as needed to maintain the desired degree of acidity.

Solutions replenished in accordance with the foregoing instructions arecapable of coating large areas of a1uminum surfaces with a remarkabledegree of uniformity both as to color and corrosion resistance and thisin relatively short periods of time during a continuous operation. As anactual matter of fact, coating baths, replenished as described, havebeen in substantially continuous operation under conditions of extremelyheavy bath loading in the coating of literally millions of square feetof aluminum and the coatings produced have been of remarkably uniformquality throughout both as to color and corrosion resistance.

We will now cite some specific examples. Utilizing commerciallyavailable materials a bath was prepared in accordance with the practicesof the prior art and the resulting solution had the followingcomposition which, for the sake of convenience, we will refer to asExperiment I Grams Chromic acid (CrO 4.5 Sodium fluoborate (NaBF 6.0Potassium fluozirconate (K ZrF 1.65 Potassium ferricyanide (K Fe(CN)2.85

Water, to make 1 liter.

The foregoing bath, as initially prepared, had a pH of 1.5 and was usedto coat 38 aluminum alloy pieces utilizing a 1-minute immersion time atroom temperature. The bath produced strongly corrosion resistantcoatings having a beautiful golden brown color. Subsequently, a seriesof aluminum coils was processed in rapid succession through this bathalso at room temperature until a total of 40 ft? of aluminum surface hadbeen coated. The coating solution was analyzed and replenishment waseffected after every 10 square feet of aluminum had been coated and thisreplenishment was carried out in accordance with the manufacturer'sdirections and the practices of the prior art and at the same time thepH was adjusted with suitable additions of nitric acid to maintain thesame at substantially its original value. However, by the time that the40 ft? of aluminum surface had been processed and even after only fourreplenishments, the coatings produced by the bath had greatly diminishedin color intensity and were obviously inferior to the coatings producedby the bath as originally prepared.

To contrast the foregoing with the improvements of the present inventionanother bath was prepared of identical composition to the one givenabove which bath initially, and under the same conditions as notedabove,

produced strongly colored golden brown coatings. However, this bath wasreplenished in accordance with the teachings of the invention hereindisclosed. Specifically, for every 10 ft. of surface coated the bath wasrestored to its original hexavalent chromium content and at the sametime, for each part of chromate added, there was added 0.5 part offluoride in the form of hydrofluoric acid and 0.6 part of ferricyanide.When necessary the acidity of the bath was adjusted to its originalvalue with suitable additions of nitric acid. When the bath was handledin this way it produced uniformly colored coatings even after 100 ft? ofaluminum surface per liter of bath volume had been processed. In fact,the coating produced on the hundredth square foot was practicallyidentical with the coating first produced by the bath.

In the comparison just noted, coating weight determinations were madeand when the operation was carried out according to the practices of theprior art, after processing 40 ft. it was found that the fortieth squarefoot of surface had a coating weight which was approximately 23% lessthan the coating weight originally produced by the bath, whereas, whenoperating according to the method of the present disclosure, the coatingweight on the hundredth square foot was practically identical to thecoating weight produced on the first square foot.

Still further examples of the process of the present invention are foundin the following:

Experiment [I An aluminum coating bath was prepared containing thefol-lowing constituents:

Water, to make 1 liter.

This bath, as initially prepared had a pH of 1.5 and was used to coat 38aluminum alloy panels using a 1-minute immersion cycle at roomtemperature. Panels treated initially were uniformly golden brown incolor and had coating weights of mg./ft.

A series of aluminum coils was subsequently processed in rapidsuccession through this bath at room temperature until a total of ft. ofaluminum surface area had been coated. The coating solution was analyzedand replenished every 20 sq. ft. with CrO a mixture of NaF and KP in themolar ratio of 2 parts K to 1 part Na and K Fe(CN) The ratios of thereplenishing materials were:

CrO :F-(asNaE+2KF mixture) :K Fe(CN) ='1:1:0.12

The pH was adjusted to the desired 1.5 to 1.7 range using smallincrements of concentrated nitric acid. Aluminum panels coated for 1minute in this bath, replenished in the above manner, were found to bequite consistent in color uniformity. Coating weights from start tofinish, as well as color, were substantially uniform.

Experiment III An aluminum coating bath was prepared containing thefollowing constituents:

Grams Chromic acid (CrO 5.0 Potassium ferricyanide (K Fe(CN) 0.6Hydrofiuoric acid (HF) 1.05

Water, to make 1 liter.

This bath, as initially prepared had a pH of 1.5 and was used to coat 35aluminum panels using a 1-minute immersion cycle at room temperature.Panels treated initially were uniformly golden brown in color and hadcoating weights of approximately 80 mg./ft.

A series of aluminum coils was subsequently processed in rapidsuccession through this bath at room temperature until a total of 100ft. of aluminum surface area had been coated. Analysis was made of thecoating solution and replenished every 20 sq. ft. with CrO F (as HF) andK Fe(CN) in the ratio of 1:0.5:0.12 by weight. No nitric acid was addedduring this run since the pH was found to have remained constant. Thefinal pH was 1.5. Aluminum panels coated for one minute in this bathreplenished in the above manner were found to have remained quiteconstant in color uniformity.

We claim:

1. In the production of chemical conversion coatings on a succession ofaluminum surfaces where the surface is treated with an aqueous acidsolution consisting essentially of a fluorine bearing compound andhexavalent chromium, the method of maintaining the solution in coatingcondition throughout its operating life which comprises periodicallyrestoring the hexavalent chromium content of the solution to the desiredlevel and replenishing fluorine by adding fluoride from the classconsisting of hydrofluoric acid and alkali and ammonium salts thereof,the quantity of fluoride from said class which is added being from 0.4to 2 parts for each part of chromate (calculated as CrO 2. The method ofclaim 1 wherein the fluoride is added as hydrofluoric acid in an amountof from 0.5 to 0.8 part for every part of chromate added.

3. The method of claim 1 wherein the fluoride is added as salt from thesaid class in an amount of from 0.75 part to 1.25 parts for each partchromate added.

4. The method of claim 1 wherein the solution also contains ferricyanideas an addition agent and wherein said agent is replenished by addingfrom 0.05 to 1 part thereof for each part of chromate added.

5. The method of claim 1 wherein the solution also contains ferricyanideas an addition agent and wherein said agent is replenished by addingfrom 0.2 to 1 part thereof for each part of chromate added.

References Cited in the file of this patent UNITED STATES PATENTS2,507,956 Bruno et al May 16, 1950 2,796,371 Ostrander et a1 June 18,1957 2,825,697 Carroll et al. Mar. 4, 1958

1. IN THE PRODUCTION OF CHEMICAL CONVERSION COATINGS ON A SUCCESSION OFALUMINUM SURFACES WHERE THE SURFACE IS TREATED WITH AN AQUEOUS ACIDSOLUTION CONSISTING ESSENTIALLY OF A FLUORINE BEARING COMPOUND ANDHEXAVALENT CHROMIUM, THE METHOD OF MAINTAINING THE SOLUTION IN COATINGCONDITION THROUGHOUT ITS OPERATING LIFE WHICH COMPRISES PERIODICALLYRESTORING THE HEXAVALENT CHROMIUM CONTENT OF THE SOLUTION TO THE DESIREDLEVEL AND REPLENISHING FLUORINE BY ADDING FLUORIDE FROM THE CLASSCONSISTING OF HYDROFLUORIC ACID AND ALKALI AND AMMONIUM SALTS THEREOF,THE QUANTITY OF FLUORIDE FROM SAID CLASS WHICH IS ADDED BEING FROM 0.4TO 2 PARTS FOR EACH PART OF CHROMATE (CALCULATED AS CRO3).